Catalytic systems for automotive emission control have usually been either of the loose bed type or of the structured support type. The loose bed type is often referred to as a non-supported catalyst because the catalytic ingredients are formed as discrete pellets grouped together in a bed forming a network of passages between such pellets. The loose bed of catalytic system has proved to be inefficient for automotive use because of the need for increased efficiency within a minimum space allotment.
The structured support type of catalytic system typically has a preformed unitized matrix upon which is deposited the catalytic chemicals. This type of system has proved somewhat more desirable in that the resistance to flow of exhaust gases therethrough can be reduced considerably by proper design of the matrix; the matrix can be constructed of a bundle of thin wires or ribbons, or fused plys of ceramic material interleaved in a predetermined pattern, or a cast ceramic honeycomb matrix. More importantly, the catalytic chemicals can be deposited thereon in a very thin layer reducing the employed catalytic amount and reducing the cost of construction.
Although ceramic matrices have been employed, they have suffered somewhat from the problems of being fragile and subject to breakage resulting from thermal expansion when utilized in a high temperature environment for a long period of time. Developmental efforts of the art has therefore turned to metallic supported catalyst systems to obtain greater efficiency, economy and durability for automotive catalytic systems. The problems encountered in the development of metallic supported catalyst systems are reviewed chronologically below.
The first problem encountered and overcome was the need for separation of the catalyst material from the metal substrate so that, under high temperature use, the catalyst material would not become inactive by thermal diffusion into the metallic substrate. The obvious approach to solve this problem was to interpose a layer of inert material (alumina or other ceramic) between the sheet substrate (usually alloy steel) and the exposed catalytic material (such as platinum); see further U.S. Pat. No. 3,437,605. Unfortunately, the deposited ceramic coating upon the steel substrate peeled and debonded during use, reducing catalytic function. The prior art attempted to solve this peeling problem by requiring that the catalytic material, itself, be one that would not become inactive by thermal diffusion. To this end, one approach was to promote a conversion coating of gamma alumina on the steel substrate. This was carried out by dipping the metallic substrate in an oxidizing acid (such as nitric acid) to dissolve iron; the substrate was then dipped in a metal hydroxide solution and thereafter dried and exposed to a hot oxygen environment to convert the aluminum hydroxide to gamma alumina. Unfortunately, with this system it was necessary to employ metal disc supports that were 0.35-0.45 millimeters thick and were required to be retained between screens without intimate bonding. This system suffered from lack of catalytic efficiency and economy of manufacture. (See U.S. Pat. No. 3,711,856).
Another approach to selecting a catalyst which would not be inactivated by thermal diffusion was to employ monel metal (Ni-Cu) bonded to stainless steel; the intimately bonded pieces were used as the total catalytic system. Stainless steel, unfortunately could only be supplied in a thickness above 0.03 inches which prevented good flow efficiency (see U.S. Pat. No. 3,733,894).
To advance the prior art further, it was next suggested that successive layers be employed with the final layer being an optimum rare earth type catalytic material, such as platinum. This, of course, required some attention to providing better adherency between the layers of the system and also to increasing the surface area supporting the platinum coating for increased efficiency. One approach by the prior art, in accordance with this concept, employed a stainless steel substrate, typically in the form of wire (the stainless steel having a composition of about 15% nickel and 22% chromium). The substrate was dipped in an aluminum silicate solution and then sprayed with a catalyst carrier material, such as aluminum hydroxide; the aluminum hydroxide was thereafter impregnated with the catalyst material, such as platinum. The total coated system was calcined at an appropriately high temperature to convert the silicates to an aluminum oxide-silicate mixture (see U.S. Pat. No. 3,891,575).
Another multi-layer approach was to coat a stainless steel substrate (0.1 mm thick) with aluminides (such as nickel aluminide) and spray alumina thereover; the coated system was then chemically activated by heating at 500.degree. C. to produce a converted alumina coating. The alumina coated substrate was then dipped in a suitable solution containing platinum. The substrate pieces were not bonded, but rather mechanically held together in an envelope or held by fasteners; this limited the ability of the assembly to have low resistance to flow (see U.S. Pat. No. 3,907,708). Each of these approaches was not sufficiently satisfactory with respect to adhesion between layers and oxidation resistance of the metallic substrate.
Paralleling this development of the metallic supported catalyst system for automotive use, was the development of high temperature oxidation resistant metals useful in nuclear reactor systems. Certain Fe-Cr-Al systems were developed which gave favorable oxidation resistance due to the formation of a tenacious aluminum oxide (alumina) film thereon (see U.S. Pat. Nos. 3,027,252; 3,298,826; and 3,867,313). To increase the retention of the oxide film, yttrium was added to form an intermetallic phase with the iron promoting a tightly adhered oxide layer; unfortunately the yttrium additive adversely affects the cost of the process.
The Fe-Cr-Al alloy has been proposed for use as metallic substrate in catalytic systems. Although not the earliest use or proposal, U.S. Pat. No. 3,920,583 evidences such use of the substrate. The substrate is formed as a matrix for a catalytic system, containing up to 12% chromium, 0.5-12% aluminum and 0.1-3% yttrium, and the balance iron; it is subjected to heat in a flowing oxygen environment whereby the surface of the substrate becomes chemically active and is converted to an alumina coating, the alumina coating growing out of the metal itself; the heat treated substrate is then coated with a catalytic material such as platinum.
In spite of the advanced stage of both lines of development, there remains to date several significant problems that must be overcome for satisfactory commercial use of a metallic supported catalyst system in automotive vehicles. These problems comprise: (a) a need for even greater reduction in the resistance to flow of the matrix, (b) a need for a faster heat-up rate of the mass of the matrix and the catalytic material so as to become more quickly operative during start-up conditions, (c) improving the rigidity and bonding characteristics of the different parts of the matrix for an increased durability over longer periods of use, (d) the high cost of fabrication, and (e) a need for improving oxidation resistance of the substrate material without the necessity for using exotic materials such as yttria. The prior art to date has failed to produce a matrix material thin enough to provide a wall thickness that offers less resistance to flow. Because of the relative thickness of the matrix walls, the mass of the material remains relatively high for a given volume of a catalytic structure. Most of the commercially available catalyst systems have the substrate retained mechanically in place to maintain a predetermined configuration and thus can be considered unbonded or non-welded. This lack of rigidity has lead to a decrease in longevity of the device. The use of complicated chemical coating systems and treatment steps, as well as the use of expensive materials, has lead to very high fabrication costs. But the most critical barrier that must be overcome with respect to the wall sections, is the ability to roll the metallic substrate to a much finer gauge with ease while at the same time affording increased chemical protection against oxidation. The use of high chromium content steel alloys has limited such ability to roll to thin gauges because of the work hardening and embrittlement that takes place; an inordinate number of annealing cycles must be employed, requiring annealing between many successive reductions thereby increasing costs.